The successful operation of a hearing prosthetic implant depends in part on the ability to convey pitch information. Differing pitch percepts may be produced by a hearing prosthetic implant in two distinct ways. Firstly, electrical stimulation at different sites in the cochlea excites different groups of neurons and because of the tonotopic arrangement of neurons in the cochlea, different pitch sensations result. By tonotopic is meant that the percept corresponding to a particular site in the cochlea changes in pitch from lower to higher as the site is changed in an apical to basal direction. Pitch varied in this way is known as “place pitch”. Secondly different pulse rates of electrical stimulation produce different pitch sensations. Pitch varied in this way is known as “rate pitch”. The current invention deals with various means of varying place pitch.
There are several commonly known methods or modes of stimulation. Each of these different known modes of operation has traditionally been employed using only one mode in each patient.
One of the current known modes used is bipolar stimulation which generally comprises passing biphasic current pulses between pairs of electrode bands, for example on a 22-band electrode array. The bands may be numbered from 1 at the basal end (broad part of the cochlea spiral, corresponding to high frequencies, near the entry point of the array) to 22 at the apical end (somewhere in the middle of the spiral, corresponding to lower frequencies). In its general sense, bipolar stimulation may involve passing current flows between any two bands. The term bipolar is also used in a specific sense to describe stimulation using two bands adjacent one another (e.g. 3 and 4) if the foregoing numbering system is utilised. More commonly, a bipolar+1 system is utilised where current flows between a band and the next but one band (e.g. 3 and 5). Similarly, “bipolar+n” systems are known where current flows between electrode k and electrode k+n+1.
Another known mode of stimulation is “monopolar” or “remote ground”. In the following, when reference is made to this system, the term “monopolar” is used. In this case, current flows between one electrode inside the cochlea (possibly one of the 1 to 22 electrodes in the known system discussed above) and a single electrode somewhere outside the cochlea. This arrangement has some advantages including lower current requirements but, until now, a perceived disadvantage in precision with which current may be delivered.
Another known mode of stimulation employs a single electrode, possibly one band on the bared array (22-band array discussed above) as a return electrode for all stimuli, but different first bands (e.g. 3,1; 4,1; 5,1 . . . 22,1). This is a particular type of bipolar (referred hereinafter as “variable bipolar”) stimulation. If the return electrode is inside the cochlea it is similar to bipolar stimulation but is very similar to monopolar stimulation if the return electrode is outside the cochlea.
A still further arrangement referred to as “common ground” uses a variable first electrode and all the other electrodes are connected together as a return.
Generally, as the stimulating electrode changes (in the above-mentioned 22 band array), or in other words the site of stimulation in the cochlea, the pitch sensation changes regularly and monotonically (from high for low numbered electrodes to low for high numbered electrodes), as would be expected from the tonotopic arrangement of the cochlea. However, this is not always the case, and the pitch sensations perceived for the different modes described above are different.
The current distribution associated with stimulation of a given electrode site may also be varied by the type of electrode in the array, or more relevantly, the geometry of the electrode. There are various geometries disclosed in the prior art, including band electrodes, and sided arrays with conductive material only on part of the array surface. Use of different electrode geometries will also produce different current distributions around the site of the stimulus. In known arrangements, various electrode geometries have been used for differing purposes, however, the geometry of the intracochlear electrodes in any particular patient has been the same.
The objective of the present invention is to provide an improved arrangement for use in hearing prosthetic implant devices and an improved method of stimulating electrodes in such devices that will result in better speech perception in patients.